The present invention relates to a rear derailleur for bicycle gear changes.
Bicycle derailleurs comprise a top body which is designed to be fixed to the bicycle frame and which is referred to as “hanger bracket”, and a bottom body, referred to as “pulley-cage bracket”, which carries a rocker member that in turn carries two idler wheels that co-operate with the chain. The pulley-cage bracket is connected to the hanger bracket by means of an articulated-parallelogram mechanism comprising a first link and a second link. The said links are articulated to the hanger bracket and the pulley-cage bracket by means of pins which define the axes of articulation of the articulated-parallelogram mechanism.
In higher-quality derailleurs, the hanger bracket, pulley-cage bracket and the links of the articulated-parallelogram mechanism are made of light metal alloys, such as aluminium alloys or the like. Articulation of the links of the articulated-parallelogram mechanism to the hanger bracket and pulley-cage bracket is obtained by inserting axially the pins inside aligned holes of one of the brackets and one of the links. Each pin is constrained against sliding out in the direction of its own axis by means of plastic deformation of one or both of its ends against a surface of the hanger bracket or of the pulley-cage bracket, or else by means of a knurling of the pin which interferes with a corresponding wall of a hole made in one of the bodies. The deformation of the ends of the pins against the hanger bracket or the pulley-cage bracket, or else the drive fit of a knurled portion of the pin in a hole, does not create any problem in the case where the said bodies are made of metal material. It has, however, been found that the traditional method of axial constraint of the pins gives rise to problems of initiation of failure or cracking in the case where the hanger bracket or pulley-cage bracket is made of plastic material.